1. Field of the Invention
This invention relates to multiple tape thickness, multiple recording format tape drive systems. More particularly, the invention relates to the ability of a tape system to distinguish between different tape thicknesses and recording formats.
2. Description of the Related Art
Modern tape drive systems use a reel-to-reel drive to move a tape back and forth in close proximity to one or more read and/or write transducing heads. In some tape systems, certain characteristics of a tape cartridge are determined and used to improve system performance. For example, many video tape systems are capable of recording at different tape speeds and, to enable proper playback, of determining the speed at which such recording occurred. Also, some tape systems determine the amount of tape wound upon a tape reel. For example, U.S. Pat. Nos. 4,644,436, 4,280,159, and 4,237,371 describe measuring the amount of tape on a reel to enable the system to indicate the remaining recording and/or playback time. U.S. Pat. Nos. 4,644,436 and 4,280,159 also infer the thickness of a tape from amount of tape measured on a reel. U.S. Pat. Nos. 4,541,027 and 4,389,600 describe measuring the amount of tape wound on a reel to adjust the torque applied to the tape drive motors to maintain proper tape tension during tape drive operation.
The length and format of a tape are also determined in the IBM 3480 and 3490 Magnetic Tape Subsystems (hereinafter referred to simply as "old" IBM tape systems for convenience). Upon insertion of a single reel tape cartridge into a tape drive, a leader block attached to the longitudinal end of the tape is automatically threaded through the tape path and the tape is spooled upon the take-up reel. During such threading and spooling, the length of tape wound upon the supply reel (the single reel tape cartridge inserted into the tape drive) is determined using tachometers on the supply and take-up reels. The tachometers are used to measure the amount of rotation of such reels during a specified period. The amount of rotation of the supply reel during one or more rotations of the take-up reel (a physical measure of tape circumference) is inversely related to the amount or length of tape on the supply reel. Thus, the length of tape wound upon the supply reel can be determined, without actually measuring the entire length of the tape, using well known mathematics. The length of tape measurement is used to properly maintain a constant tape tension for the most reliable system operation, and for rejecting out of specification tape cartridges.
Upon threading of the tape in an old IBM tape system, a deformation is formed in the tape as it wraps upon itself on the take-up reel. Depending upon the conditions, the deformation may become embossed in the tape. Reliable reading and/or writing may not be possible at or near such a deformation. The old IBM tape systems record user data in 18 parallel tracks, as is known, including two 8-bit bytes and two parity bits (one parity bit for each byte) across the tape width. Because each bit is recorded at a single tape location, the lack of a smooth recording surface at the head-tape interface can result in the irreparable loss of one or more bits. Fortunately, the deformation decreases with each successive wrap of the tape upon the take-up reel. The amount of tape required to be wrapped upon the take-up reel to decrease the deformation to a size small enough to permit reliable read/write activity can be determined experimentally. Thus, enough tape is spooled upon the take-up reel to ensure that the recording of data does not begin until such can occur reliably.
In an old IBM tape system, the point on a tape at which the recording of user data is first permitted is referred to as the logical beginning of tape (LBOT). For ease of understanding, LBOT is hereinafter referred to as the beginning of user data (BOD). The location of the BOD is defined by the tape system control unit. In an old IBM tape system, the first user data recorded after the BOD is customarily the volume serial number (VOLSER). A volume is a physical data unit for convenient handling. In a tape system, the data recorded on a single tape cartridge is considered to be a volume as it can be conveniently mounted in a tape drive.
A format identifier (FID) is a pattern recorded on a tape prior to the BOD. The specific pattern used, also referred to as the "type" of FID, is indicative of the format of data recorded (or to be recorded) after the BOD. The point on a tape where the FID begins is referred to in an old IBM tape system as the physical beginning of tape (PBOT). For ease of understanding, PBOT is hereinafter referred to as the beginning of format identifier (BOF). The BOF is defined in the tape device, not the tape system control unit, as a specified distance from the tape leader block. Although a FID is a form of recorded data, it is not user data, and can be reliably written to and read from the tape prior to the BOD, even in proximity to a deformation, because of the uniqueness of the recorded pattern. The old IBM tape systems record a FID as a unique 6-bit tone recorded on 9 of the 18 tracks in the tape width. The 6-bit tones are divided into 3 sets of 3 tracks each and are distributed across the 18-track tape width; each set of 3 tones being continuously repeated along the length of the tape between the BOF and the BOD. The tones in each set of 3 tracks can be compared until sufficient matching occurs to reliably identify the tone, and hence the type of FID, despite the existence of a deformation in the tape somewhere between the BOF and the BOD. The type of FID is therefore determined by the tape system, upon tape cartridge mounting, prior to the BOD being positioned in proximity to the transducing head.
By identifying the type of FID, a tape system can signal an operator to remove a tape cartridge therefrom if the format of data recorded on the tape is not compatible therewith.
The lack of a FID normally indicates a blank volume. When no FID is detected in an old IBM tape system, the tape is rewound from the BOD to the BOF and a FID is recorded therebetween upon receiving a command from the host processor to write user data, such as the VOLSER, after the BOD. Upon subsequent mounts of the tape, the proper FID should be identified. Should the host processor command that a tape be written or rewritten from the BOD (such as the VOLSER), the system writes the FID again. Should the host processor command that the tape be rewound, such an operation occurs under the control of the tape device, not the tape system control unit. Tape device control permits the control unit to logically disconnect from the tape device to direct other tape system operations. Because the BOF, and not the BOD, is known to the tape device, such tape device control results in the tape being rewound to the BOF. To access user data on the tape, the tape must subsequently be advanced past the BOD. The aforementioned tape movement between the BOF and the BOD significantly impacts tape system performance.
The IBM 3490E Magnetic Tape Subsystem (hereinafter referred to simply as the "new" IBM tape system for convenience) recently became available. This system records data in 36 parallel tracks using a 36-track interleaved read/write transducing head. Data is written in two sets of 18 interleaved tracks, including the same number of bits and bytes per set as in the 18-track format of an old IBM tape system. Beginning at a first longitudinal end of a tape, the first set of 18 tracks of data are written in a first direction of tape movement until the opposite end of the tape is approached. The direction of tape movement is then reversed and the second set of 18 tracks are written, interleaved among the first set of tracks, until the first end of the tape is again approached.
The old and new IBM tape systems currently employ the same "old" magnetic tape cartridge, including 1/2 inch wide magnetic tape. To achieve the increased number of tracks, a track written by a new IBM tape system is significantly narrower than a track written by an old IBM tape system. Data written by an old IBM tape system can be read by a new IBM tape system because the tracks are wider than the read/write elements of the new IBM tape system transducing head. However, user data written by a new IBM tape system cannot be read by an old IBM tape system because the tracks are too narrow to be reliably sensed by the read/write elements of an old IBM tape system transducing head. A different type of FID is written by the old and new IBM tape systems to distinguish data written in the 18-track format from data written in the 36-track format. The FID types are used by the tape systems to reject operations incompatible with an old tape cartridge mounted therein, thereby ensuring that all data written on a tape is in the same format.
The new IBM tape system writes a FID in a such a way as to enable the old IBM tape system to read such 36-track FID. The new IBM tape system writes the FID tones for the 36 track format on one set of 18 tracks as does the old IBM tape system. In addition, the new IBM tape system writes logical zeroes on the other set of 18 tracks (with the direction of tape movement reversed) from beyond the BOD to the BOF. The logical zeroes on the other set of 18 tracks are recognized as mere noise by the 18 track transducing head of an old IBM tape system during reading of the 36-track FID.
In the future, a "new" and currently unavailable magnetic tape cartridge may be marketed for use in the new IBM tape system. The new tape cartridge includes a cartridge shell similar to that of the current tape cartridge, but also includes a significantly greater length of tape therein. The increased tape length improves the data storage capacity of the new tape cartridge relative to the old data cartridge. To achieve the greater length of tape, the thickness of the tape in the new magnetic tape cartridge is reduced to allow for additional wraps thereof about the tape cartridge hub. Hereinafter, the tape in the old tape cartridge, or any other relatively thick tape, is referred to as "thick" tape and the tape in the new tape cartridge, or any other relatively thin tape, is referred to as "thin" tape. Unfortunately, the reduced thickness of thin tape renders it susceptible to perturbations which cannot be accommodated in an old IBM tape system, thereby preventing reliable read/write operations To upgrade an old IBM tape system to accommodate thin tape would require the costly replacement of certain components, eg, to adjust the tape tension therein. In addition, a heretofore unrecognized problem is how to make both thick and thin tapes compatible with the new IBM tape system. Operators might accidentally mount a tape cartridge in a tape system and request a desired action which is incompatible therewith.
Some tape systems are compatible with multiple tapes of different lengths or thicknesses. The length or thickness of a tape is generally of little significance to its compatibility with a particular tape system. The compatibility of a tape with a particular tape system is therefore not determined by the thickness or length of tape therein. In addition, the inherent nature of the recorded data in many such tape systems (audio or video signals in which a loss of a small portion of the signal is not critical) renders them much less susceptible to the aforementioned problem of deformations in a tape for use in a computer system. Thus, a heretofore unrecognized problem is how to accommodate multiple tape thicknesses and recording formats in tape systems.